Giignl cthb 5.0.web_

GIIGNL
LNG CUSTODY TRANSFER
HANDBOOK
5th
Edition: 2017

2GIIGNL - LNG CUSTODY TRANSFER HANDBOOK Fifth edition – version 5.0 – Uncontrolled when printed
Document status and purpose
This fifth (2017) edition of the GIIGNL LNG
Custody Transfer Handbook reflects GIIGNL’s
understanding of best current practice at the
time of publication.
The purpose of this handbook is to serve as a
reference manual to assist readers to
understand the procedures and equipment
available to and used by the members of GIIGNL
to determine the energy quantity of LNG
transferred between LNG ships and LNG
terminals. It is neither a standard nor a
specification.
This handbook is not intended to provide the
reader with a detailed LNG ship-shore custody
transfer procedure as such, but sets out the
practical issues and requirements to guide and
facilitate a skilled operator team to work out a
suitable procedure for a specific LNG ship-shore
custody transfer application.
The fifth version comes as a quick follow-up and
update of the fourth edition (2015) which
provided, for the first time, a useful insight in
the relatively new (commercial) opportunities
resulting from the rapidly changing market
conditions: new technical solutions and different
operations such as partial (un)loading, reloading
at LNG import terminals, ship-to-ship LNG
transfer and LNG transfer from an onshore
terminal to small scale LNG carriers. More than
pointing at the differences and highlighting the
points of attention when dealing with these new
operations, this fifth version provides answers
and solutions for setting up (slightly) altered or
new custody transfer procedures. As a reminder,
it is not specifically intended to work out
procedures for overland LNG custody transfer
operations involving LNG trucks, containers or
trains, or for small scale LNG transfer such as
bunkering or refueling of ships and trucks. For
these, kind reference is made to the GIIGNL
Retail LNG / LNG as a fuel handbook.
No proprietary procedure, nor particular
manufacture of equipment, is recommended or
implied suitable for any specific purpose in this
handbook. Readers should ensure that they are
in possession of the latest information,
standards and specifications for any procedures
and equipment they intend to employ.
GIIGNL, and any of its members, disclaims any
direct or indirect liability as to information
contained in this document for any industrial,
commercial or other use whatsoever.
This latest version replaces all previous editions
of the custody transfer handbook. Please always
consult the GIIGNL website www.giignl.org to
check for the latest version of this handbook,
esp. when referring to a pdf download or a
printout of this handbook
(Photo front cover : © Fluxys Belgium – P. Henderyckx)

Contents
1. Introduction.......................................................................................... 7
2. General description of the measurement .......................................... 11
2.1. GENERAL FORMULA FOR CALCULATING THE LNG ENERGY TRANSFERRED.....11
2.2. GENERAL SCHEME OF THE MEASUREMENT OPERATIONS ..............................12
2.2.1. LNG Volume..............................................................................................13
2.2.2. LNG Density ..............................................................................................14
2.2.3. LNG Gross calorific value..........................................................................14
2.2.4. Energy of the gas displaced by the transfer of LNG .................................14
2.3. INSTRUMENTS USED........................................................................................16
2.3.1. For the determination of the LNG volume ...............................................16
2.3.2. For the determination of LNG density and gross calorific value ..............16
2.3.3. For the determination of the energy of displaced gas .............................17
2.3.4. For the determination of the energy of “Gas to engine room”................17
2.3.5. Periodic instruments recalibration ...........................................................18
2.4. STANDARDIZATION..........................................................................................18
2.5. PARTIAL LOADING OR UNLOADING OF LNG CARRIERS ...................................18
2.6. Gassing-up and cooling down operations........................................................18
2.6.1. Gassing-up operations..............................................................................18
2.6.2. Cooling down operations..........................................................................19
3. Volume measurement........................................................................ 22
3.1. GAUGE TABLES.................................................................................................22
3.1.1. Use of gauge tables ..................................................................................22
3.1.2. Correction tables ......................................................................................22
3.1.3. Approval by authorities ............................................................................23
3.2. INSTRUMENTS AND METHODS FOR MEASURING THE LEVEL OF LIQUID IN THE
LNG CARRIER'S TANKS............................................................................................ 25
3.2.1. Main liquid level gauging devices ............................................................ 25
3.2.2. Timing of the level measurement ............................................................ 27
3.2.3. Readings................................................................................................... 27
3.2.4. Correction of readings ............................................................................. 28
3.2.5. Use of spare level gauge .......................................................................... 32
3.2.6. Complete unloading (tank stripping) ....................................................... 32
3.2.7. Automated systems ................................................................................. 33
3.3. CALCULATION OF THE VOLUME OF LNG TRANSFERRED................................. 34
4. Temperature measurement................................................................36
4.1. LIQUID TEMPERATURE .................................................................................... 36
4.1.1. Device....................................................................................................... 36
4.1.2. Testing and accuracy................................................................................ 36
4.2. VAPOR TEMPERATURE .................................................................................... 37
5. Vapor pressure....................................................................................38
6. Sampling of LNG..................................................................................39
6.1. LNG SAMPLING PRINCIPLES ............................................................................ 39
6.2. SAMPLING POINT ............................................................................................ 40
6.3. SAMPLING PROBES.......................................................................................... 41
6.4. PIPING ARRANGEMENT BETWEEN SAMPLING PROBE AND VAPORIZER ........ 42
6.5. LNG VAPORIZER AND CONTROL DEVICES ....................................................... 42
6.5.1. Main devices ............................................................................................ 42
6.5.2. Auxiliary vaporization control devices ..................................................... 45
6.5.3. Operating parameters.............................................................................. 46

Contents
6.5.4. Gas sample collection systems .................................................................46
6.6. GAS SAMPLE CONDITIONING...........................................................................47
6.6.1. Gas sample containers..............................................................................47
6.6.2. Direct piping to a gas analyzer..................................................................47
6.6.3. Examples...................................................................................................47
6.7. PERFORMANCE OF THE DEVICES .....................................................................51
6.8. SAMPLING PROCEDURE...................................................................................51
6.8.1. Sampling period........................................................................................51
6.8.2. Sampling frequency..................................................................................51
6.8.3. Purging......................................................................................................52
6.8.4. Sampling parameters................................................................................52
6.8.5. Utilisation of gas sample containers.........................................................52
6.9. SPOT SAMPLING DEVICE..................................................................................52
7. Gas Analysis ........................................................................................ 53
7.1. TYPE OF GAS CHROMATOGRAPH ....................................................................53
7.2. CALIBRATION ...................................................................................................53
7.2.1. Calibration gas/working standard ............................................................54
7.3. ENVIRONMENT FOR A GAS CHROMATOGRAPHIC SYSTEM .............................54
7.4. ANALYSIS OF REGASIFIED LNG AND RETAINED SAMPLES................................55
7.5. UNCERTAINTY OF GAS ANALYSIS .....................................................................55
7.6. RAMAN SPECTROSCOPY ..................................................................................55
7.7. IMPURITIES ......................................................................................................56
7.7.1. General .....................................................................................................56
7.7.2. Specifications and measurement of trace impurities in LNG ...................57
8. Data processing & treatment ............................................................. 60
8.1. GENERAL ..........................................................................................................60
8.2. DATA HANDLING: QUALITY ............................................................................. 60
8.2.1. Data processing........................................................................................ 60
8.2.2. Data treatment ........................................................................................ 61
9. Density.................................................................................................62
9.1. GENERAL.......................................................................................................... 62
9.2. DENSITY CALCULATION METHODS.................................................................. 62
9.3. REVISED KLOSEK-Mc KINLEY METHOD............................................................ 63
9.3.1. Limits of the method................................................................................ 63
9.3.2. Formula.................................................................................................... 63
9.3.3. Example of LNG density calculation......................................................... 63
10. Gross calorific value ............................................................................64
10.1. GENERAL........................................................................................................ 64
10.2. METHOD OF DETERMINATION OF THE GROSS CALORIFIC VALUE ................ 64
10.2.1. Determination with the help of calorimeters ........................................ 64
10.2.2. Determination of GCV by calculation..................................................... 65
11. Analysis report ....................................................................................68
11.1. IDENTIFICATION ............................................................................................ 68
11.2. BASIC DATA ................................................................................................... 68
11.3. RESULTS......................................................................................................... 68
12. Energy displaced or consumed ...........................................................69
12.1. ENERGY OF GAS DISPLACED FROM THE LNG TANKS..................................... 69
12.2. ENERGY OF GAS CONSUMED AS FUEL BY THE LNG CARRIER........................ 70
13. Energy transfer determination............................................................71
14. Energy transfer measurement ............................................................72

Contents
15. Uncertainty of the energy transfer determination ............................ 73
15.1. VOLUME.........................................................................................................73
15.1.1. Cargo Liquid Lines...................................................................................74
15.2. DENSITY..........................................................................................................74
15.3. GROSS CALORIFIC VALUE...............................................................................75
15.4. SAMPLING AND VAPORIZATION ....................................................................75
15.5. GAS DISPLACED..............................................................................................75
15.6. GAS CONSUMED IN ENGINE ROOM...............................................................75
15.7. COMBINED STANDARD UNCERTAINTY AND EXPANDED UNCERTAINTY OF
THE ENERGY TRANSFER DETERMINATION..............................................................75
15.8. ROUNDING OF NUMBERS AND COMMERCIAL IMPACT ................................76
16. Ship-to-ship transfer operations......................................................... 78
17. Loading and unloading of small scale vessels..................................... 80
17.1. APPLICATION OF GENERAL FORMULA FOR CALCULATING THE ENERGY
TRANSFERRED.........................................................................................................80
17.1.1. Volume ...................................................................................................80
17.1.2. Density....................................................................................................81
17.1.3. Gross Calorific Value...............................................................................82
17.1.4. Gas displaced..........................................................................................82
17.1.5. Uncertainty of volume measurement ....................................................83
17.2. ALTERNATIVE METHOD FOR CALCULATING THE ENERGY TRANSFERRED .....83
18. Reloading operations in regasification terminals............................... 85
19. LNG sales contract custody transfer checklist.................................... 87
20. Annexes .............................................................................................. 90
ENCLOSURE 1: CONVERSION FACTOR TABLE FOR ENERGY UNITS (1), (2)......90
ENCLOSURE 2: LNG AND NATURAL GAS CUSTODY TRANSFER METHODS......91
ENCLOSURE 3: OTHER RELEVANT STANDARDS & REFERENCES......................97
ENCLOSURE 4: NOTES ON TERMINOLOGY......................................................99
LIST OF FIGURES............................................................................................100
LIST OF TABLES..............................................................................................102
REFERENCES ..................................................................................................104
Appendix 1: USE OF IN-LINE MEASUREMENT OF LNG FLOW .......................106
Appendix 2: LASER TYPE CARGO TANK LEVEL GAUGE ..................................108
Appendix 3: SOME RECOMMENDATIONS FOR PARTIAL RELOADING OF
CARGO TANKS OF LNG CARRIERS, WITH REGARD TO BOTH SAFETY AND
CUSTODY TRANSFER ISSUES..........................................................................109
Appendix 4: EXAMPLE OF gauge tables (see Section 3)................................112
Appendix 5: EXAMPLE OF VOLUME CALCULATION (see Section 3.3)...........123
Appendix 6: EXAMPLES OF DISCONTINUOUS AND CONTINUOUS SAMPLING
AND VAPORIZATION SYSTEMS......................................................................132
Appendix 7: DIRECT IN-LINE ANALYSIS WITH RAMAN SPECTROSCOPY........134
Appendix 8: Grubbs’ test [19] and [20].........................................................136
Appendix 9: TABLES FOR LNG DENSITY CALCULATION ACCORDING TO NBS
(see Section 9)...............................................................................................144

Contents
Appendix 10: EXAMPLE OF LNG DENSITY CALCULATION (see Section 9) .... 147
Appendix 11: EXAMPLE OF GCV CALCULATION (see Section 10.2.2.3)........ 152
Appendix 12: UNCERTAINTY CALCULATIONS (See Section 15) .................... 154
Appendix 13: EXAMPLE OF COMMERCIAL IMPACT OF ROUNDING OF
NUMBERS (see Section 15.8)........................................................................ 158
Appendix 14: TWO CASE STUDIES ILLUSTRATING POTENTIAL RISKS OF
STRATIFICATION AND ROLLOVER IN LNG SHIP’S CARGO TANKS.................. 159
Appendix 15: EXAMPLE OF CUSTODY TRANSFER DATA LOGS ON BOARD A
LNG VESSEL................................................................................................... 164
Appendix 16: ENHANCED REVISED KLOSEK AND MCKINLEY METHOD (See
section 17.1.2) .............................................................................................. 166
Appendix 17: MOLAR BALANCE APPLIED FOR RELOADING OPERATIONS (See
section 18) .................................................................................................... 170

1.Introduction
1. Introduction
Following the publication in 1985 by the N.B.S.
of its study "LNG Measurement - A User's
Manual for Custody Transfer" [8], the Executive
Committee of the GIIGNL (Groupe International
des Importateurs de Gaz Naturel Liquéfié)
considered it would be useful to write a
handbook, as simple and as practical as possible,
aimed at organizations involved in the
measurement of the energy transferred in the
form of LNG in the context of a LNG purchase
and sales agreement, whether this sale be F.O.B.
[Port of loading], D.E.S. or C.I.F. [Port of
unloading].
During its session of October 1987, the General
Assembly of GIIGNL decided that this practical
handbook should be drawn up by a Study Group
comprising companies of the GIIGNL and
coordinated by Distrigas S.A (B).
The methods described in this handbook could
serve to improve existing procedures. They
could also be used in purchase and sales
agreements for the GIIGNL members and serve
as a reference in new import agreements.
This handbook is based on the measurement
methods most used by GIIGNL members.
Detailed tests of the apparatus used can be
found in "LNG Measurement Study" of N.B.S. [8].
We wish to thank the companies – BG (UK) -
Distrigas Boston (USA) - Enagas (E) - Kansai
Electric Power Co (JP) - Snam (I) - Tokyo Electric
Power Co (JP) - Tokyo Gas Co Ltd (JP) - Ruhrgas
(D) – CMS Energy Trunkline LNG (USA) for their
co-operation in producing this handbook, and
more particularly Gaz de France for drawing up
Sections 6 and 7 of this handbook and Osaka Gas
Co Ltd for co-coordinating the studies of the
Japanese companies.
SECOND EDITION, OCTOBER 2001
Following the publication of the ISO 13398:1997
standard "LNG - Procedure for custody transfer
on board ship", the GIIGNL General Assembly
requested the GIIGNL Study Group to revise the
original edition (March 1991) of this GIIGNL LNG
Custody Transfer Handbook, particularly taking
into account this new ISO standard.
All 13 sections of the original edition have been
reviewed and updated where appropriate. The
following sections have been thoroughly revised:
2. General description of the measurement
3. Volume measurement
6. Sampling of LNG
7. Gas analysis
Moreover, a new section was added:
14. LNG Sales contract custody transfer
checklist.
Worked out examples for LNG density and GCV
have been rearranged in Appendices 1 and 2.
We wish to thank all companies and
organizations and their delegates who together
contributed to this second edition, viz. (in
alphabetical order):
Advantica Technologies Ltd. (UK)
BG International (UK)
CMS Energy Trunkline LNG Company (USA)
Distrigas (B)
Enagas (E)
Gaz de France (F)
Nigeria LNG (NI)
NKKK (JP)
Osaka Gas (JP)

1.Introduction
Rete Gas Italia (I)
SIGTTO (UK)
Tokyo Gas (JP)
Tractebel LNG North America (USA)
THIRD EDITION, MARCH 2010
Since the second edition, several new
international standards and revisions of existing
international standards related to the subject of
this handbook, have been published or are
forthcoming. Also, technologies and best current
practices evolved in this past period. Therefore,
the GIIGNL General Assembly requested the
GIIGNL Technical Study Group to revise the
second edition (October 2001) of the GIIGNL
LNG Custody Transfer Handbook, with the
upcoming new ISO standard ISO 10976
“Measurement of cargoes on board LNG
carriers”, which will replace and supersede the
current ISO 13397 (1997) standard upon its
publication.
Moreover this third edition of the handbook has
been updated and revised as appropriate to
streamline it with new or revised international
standards such as ISO, EN and other standards.
These include:
 EN 437: Test Gases – test Pressures –
Appliance Categories - Edition 2003
 ISO 8943 Refrigerated light hydrocarbon
fluids — Sampling of liquefied natural gas
— Continuous and intermittent methods -
Edition 2007
 ISO 6974-6 Natural gas - Determination of
composition with defined uncertainty by
gas chromatography - Part 6:
Determination of hydrogen, helium,
oxygen, nitrogen, carbon dioxide and C1 to
C8 hydrocarbons using three capillary
columns - Edition 2002
 ISO 16664: Gas analysis - Handling of
calibration gases and gas mixtures –
Guidelines - Edition 2004
 ISO 4259: Petroleum products -
Determination and application of precision
data in relation to methods of test -
Edition 2006
 ISO/TR 24094: Analysis of natural gas -
Validation methods for gaseous reference
materials – Edition 2006
 ISO 10723: Natural gas – Performance
evaluation for on-line analytical systems –
Edition 2002
 ISO 6326-1: Natural gas – Determination
of Sulphur compounds – Part 1: General
introduction – Edition 2007
 ISO 6327: Gas analysis – Determination of
the water dew point of natural gas –
Cooled surface condensation hygrometers
- Edition 2007
 ISO 19739: Natural gas - Determination of
sulphur compounds using gas
chromatography - Edition 2004
 ISO 12213-1 -2 -3: Natural gas –
Calculation of compression factor - Edition
2006
 ISO 15112: Natural gas - energy
determination - Edition 2007
 ISO 18132-1: Refrigerated light
hydrocarbon fluids – General
requirements for automatic level gauges –
Part 1: Gauges onboard ships carrying
liquefied gases – Edition 2006
 ISO 18132-2: Refrigerated light
hydrocarbon fluids – General
requirements for automatic level gauges –
Part 2: Gauges in refrigerated-type shore
tanks – Edition 2008
 ISO/DIS 28460: Petroleum and natural gas
industries – Installation and equipment for
liquefied natural gas – Ship to shore
interface and port operations – Edition
2009
All 14 sections, enclosures and appendices of the
second edition have been exhaustively reviewed
and updated where appropriate.
We wish to thank all 20 companies and
organizations and their delegates who together
contributed to this third edition, in alphabetical
order:

1.Introduction
ActOn LNG Consulting - UK
Botas - Marmara Ereglisi – Turkey
BP – Sunbury-on-Thames - UK
Distrigas of Mass.- GDF Suez- Everett (Boston)
USA
Dragon LNG - Milford Haven – UK
Elengy - GDF Suez - Paris - France
Enagas - Spain
Exxon Mobil - Houston - TX – USA
Fluxys LNG - Zeebrugge – Belgium
Gas Natural - Madrid – Spain
GL- Noble Denton - Loughborough – UK
Kogas – Seoul – South Korea
National Grid – Grain LNG - UK
Osaka Gas – Osaka - Japan
Shell Global Solutions- The Hague– The
Netherlands
RasGas - Ras Laffan – Qatar
REN Atlantico - Sines – Portugal
Sempra LNG - San Diego - CA – USA
SGS - Belgium
SIGTTO - London – UK
Tokyo Gas - Tokyo – Japan
Total – Paris - France
FOURTH EDITION, FEBRUARY 2015
Due to the rapidly changing market conditions,
new (commercial) opportunities arise leading to
new technical solutions and different operations
(such as partial unloading, reloading at an LNG
import terminal, ship-to-ship LNG transfer
operations, development of the small scale LNG
market not only with much smaller LNG ships
and in much smaller quantities, but also with a
different ship design and other cargo
containment systems), the GIIGNL General
Assembly requested the GIIGNL Technical Study
Group to review and update the third edition
(version 3.01 – March 2011) of the GIIGNL LNG
Custody Transfer Handbook.
Furthermore there is a continuous evolution in
the LNG (sampling) technology and this fourth
edition tries to incorporate and be in line with
new or revised international standards.
All sections, enclosures and appendices have
been thoroughly reviewed, amended and
updated where appropriate. Especially Sections
6 and 7 have been adapted to streamline it with
the current best practices. To make this
handbook more readable, most of the examples
have been replaced to the appendices.
Moreover, all sections with regard to the
uncertainty of the energy determination have
been moved to Section 15 and the following
sections have been added:
 2.6 Gassing-up and cooling down
operations
 15.8 Rounding of numbers
 16 Ship-to-ship LNG transfer operations
 17. Small LNG ship to shore transfer
operations
 18. Reloading operations in
regasification terminals
With regard to these new Sections 2.6, 16, 17
and 18, the aim of this GIIGNL LNG Custody
Transfer Handbook is to integrate the specific
conditions for this special operations of gassing-
up and cooling down, for ship-to-ship LNG
transfer, for small LNG ship-to-shore transfer (or
vice versa) and for reloading operations at
regasification terminals, but not to integrate the
small scale LNG transfer operations (such as

1.Introduction
bunkering or fuelling of ships and trucks, and
filling of LNG trucks or containers).
We wish to thank all companies and
organisations and their delegates who together
contributed to this fourth edition, in alphabetical
order:
Botas – Marmara Ereglisi – Turkey
BG Group – Houston – Texas
Enagas - Spain
Fluxys LNG – Zeebrugge – Belgium
Gas Natural Fenosa – Spain
Gate Terminal – Rotterdam – The Netherlands
GDF Suez – Paris – France
National Grid – Grain LNG – UK
RasGas – Ras Laffan – Qatar
Shell Global Solutions – The Hague – The
Netherlands
Tokyo Gas – Paris – France
Total – Paris - France
FIFTH EDITION, FEBRUARY 2017
The fifth version comes as a quick follow-up and
update of the fourth edition (February 2015).
More than pointing at the differences and
highlighting the points of attention when dealing
with the relatively new operations of (un)loading
small scale vessels and reloading operations at
LNG import terminals, this fifth version provides
answers and solutions for setting up (slightly)
altered or new custody transfer procedures. It
should be highlighted that the reason for these
proposed changes is of truly technical nature
and GIIGNL considers it as its duty to inform the
LNG industry about this and its impact. Each
stakeholder involved in the LNG custody transfer
chain should determine whether or not a review
of its contractual agreement(s) may be required.
We wish to express our appreciation and thanks
to all companies and organisations and their
delegates who together contributed to this fifth
edition, in alphabetical order:
Enagas - Spain
Engie – Paris – France
Fluxys LNG – Zeebrugge – Belgium
Gas Natural Fenosa – Spain
Gate Terminal – Rotterdam – The Netherlands
National Grid – Grain LNG – UK
Shell Global Solutions –The Netherlands & US
Statoil - Norway
Tokyo Gas – Paris – France

2.General description of the
measurement
2. General description of the measurement
Accuracy
The term “measurement accuracy” is defined in
the most recent version of the International
Vocabulary of Metrology (JCGM_200:2012) as
“closeness of agreement between a measured
quantity value and a true quantity value of a
measurand”. Measurement error is defined as
measured quantity value minus a reference
quantity value.
Uncertainty, combined standard uncertainty
and expanded uncertainty
The terms “measurement uncertainty”,
“combined standard uncertainty” and
“expanded uncertainty” (see Section 15) are
used as defined in the JCGM 100:2008
document: “Evaluation of measurement data –
Guide to the expression of uncertainty in
measurement”.
2.1. GENERAL FORMULA FOR
CALCULATING THE LNG
ENERGY TRANSFERRED
The formula for calculating the LNG transferred
depends on the contractual sales conditions.
These can relate to several types of sale contract
as defined by Incoterms 2010. In case of rules
for sea and inland waterway transport, the most
commonly used are a FOB sale, a CFR sale or a
CIF sale.
In the case of a FOB (Free On Board) sale, the
determination of the energy transferred and
invoiced for will be made in the loading port.
There is another sale contract similar to FOB,
named FAS (Free Alongside Ship).
In the case of a CIF (Cost Insurance & Freight) or
a CFR (Cost and Freight) sale, the energy
transferred and invoiced for will be determined
in the unloading port.
Other rules exist for any mode of transport
which can hence also apply for maritime
transport such as DAT (Delivered At Terminal)
and DAP (Delivered At Place).
In FOB contracts, the buyer is responsible to
provide and maintain the custody transfer
measurement systems on board the vessel for
volume, temperature and pressure
determination and the seller is responsible to
provide and maintain the custody transfer
measurement systems at the loading terminal
such as the sampling and gas analysis. For CIF
and CFR (and DES according to Incoterms 2000)
contracts the responsibility is reversed.
Both buyer and seller have the right to verify the
accuracy of each system that is provided,
maintained and operated by the other party.
The determination of the transferred energy
usually happens in the presence of one or more
surveyors, the ship’s cargo officer and a
representative of the LNG terminal operator. A
representative of the buyer can also be present.
In all cases, the transferred energy can be
calculated with the following formula:
  displacedgasLNGLNGLNG EGCVDVE  
±Egas to ER , if applicable
where:
E = the total net energy transferred
from the loading facilities to the
LNG carrier, from the LNG carrier
to the unloading facilities or from
one LNG carrier to another LNG
carrier (ship-to-ship LNG
transfer). In international LNG
trading, the energy transferred is
most frequently expressed in

measurement
millions of British Thermal Units
(106
BTU or MMBTU) although
this is not a SI energy unit.
Therefore, MMBTU is the
preferred unit in this handbook.
A conversion factor table for
other commonly used energy
units (such as MWh) can be
found in ENCLOSURE 1.
LNGV = the volume of LNG loaded or
unloaded in m3
.
LNGD = the density of LNG loaded or
unloaded in kg/m3
.
LNGGCV = the gross calorific value of the
LNG loaded or unloaded in
MMBTU/kg. The gross calorific
value is generally used in
international LNG trading rather
than the net calorific value, see
ENCLOSURE 4.
displacedgasE = the net energy of the displaced
gas, also in MMBTU, which is
either:
- sent back by the LNG carrier
to shore or to another LNG
carrier when loading (volume
of gas in cargo tanks displaced
by same volume of loaded
LNG),
- or, gas received by the LNG
carrier in its cargo tanks when
unloading in replacement of
the volume of discharged LNG.
ERtogasE = if applicable, the energy of the
gas consumed in the LNG
carrier’s engine room (also
including all gas burnt by the
ship’s GCU (Gas Combustion
Unit)) during the time between
opening and closing custody
transfer surveys, i.e. used by the
vessel during the LNG transfer
operation, which is:
+ for an LNG loading transfer or
- for an LNG unloading transfer
For simplicity, the parties may also make a
commercial decision to mutually agree a fixed
gas quantity/volume to estimate Egas displaced
and/or Egas to ER.
2.2. GENERAL SCHEME OF
THE MEASUREMENT
OPERATIONS
The objective is to measure the quantity of
energy loaded from production facilities into an
LNG carrier, or unloaded from an LNG carrier to
a receiving terminal. For ship-to-ship operations,
the objective is to measure the quantity of
energy transferred from one LNG carrier to
another LNG carrier.
From the above formula, it can be inferred that
five elements must be measured and/or
calculated:
- LNG volume,
- LNG density,
- LNG gross calorific value,
- energy of the gas displaced during the
transfer of LNG,
- energy of any gas consumed in the LNG
carrier’s engine room during (un)loading
operations.
A graphic overview of the measurement scheme
is shown in the figure “Flowchart for
determining the energy transferred” (see the
flowchart in Section 3.2.7)

measurement
2.2.1. LNG Volume
The standard method chosen for measuring the
volume of LNG transferred is based on the LNG
carrier's instruments, mainly the use of level
gauges and calibration tables.
For most of the vessels, gauging has become
automated via the LNG carrier’s custody transfer
measurement system. These systems are
capable of drawing up reports of the volume of
LNG on board at any time during (un)loading.
This is achieved by converting the measured LNG
levels in each cargo tank into the corresponding
LNG volume in the cargo tank via the level-to-
volume conversion tables and by applying
correction factors for trim, list and temperature
and then by totalling the volumes in all the
individual cargo tanks. Further details are given
in Section 3.2.7.
Usually a quantity of LNG, called a 'heel',
remains on board after unloading so as to keep
the tanks cold. However, operators may
sometimes prefer to strip out the cargo tanks
partially in order to maximize the LNG delivery
or totally before the LNG vessel is scheduled for
dry-docking.
Determination of the volume transferred
requires two sets of measurements, an initial
one before starting loading or unloading and a
final one at the end of the procedure. These are
called the opening and closing custody transfer
surveys (CTS) respectively. Two LNG volumes
result and the difference between the larger
volume and the smaller volume represents the
volume of liquid transferred.
For an accurate volume measurement it is
recommended that LNG piping on the LNG
carrier’s deck including manifolds be in an
identical inventory condition during both
custody transfer surveys (CTS). The piping should
either be completely filled with LNG both during
the opening custody transfer (i.e. before
(un)loading) and the closing custody transfer
(i.e. after (un)loading) or, provided that draining
is possible before the closing CTS, alternatively
be drained during both the opening and closing
CTS. Where the piping is drained before or after
the CTS measurement, it should be done for
sufficient time to fully empty the piping.
As good practice it is recommended that the
initial level gauging should be made prior to any
cooling down operation, i.e. after the
(un)loading arms have been connected but
before any ship’s liquid and vapor manifold
valves have been opened. Where the opening
CTS is conducted prior to commencement of
tank cool down, the CTS reading, where
automated, may show some liquid in the tank(s).
The system should have the capability of
‘zeroing’ such readings for level and volume,
since otherwise any liquid recorded at
commencement will be deducted from the final
CTS volume.
The final level gauging reading shall be made as
soon as possible after completion of (un)loading
with liquid and vapor arms (or flexible hoses)
drained and inerted, and with liquid and vapor
manifold valves closed.
The level gauge readings shall be determined by
the arithmetic average of several successive
readings at regular intervals. Further details are
provided in Sections 3.2.1, 3.2.2 and 3.2.3.
In the event of failure of the primary level
gauging device, an auxiliary device should be
used.
Level corrections are to be made using
correction tables provided for the LNG carrier as
tank gauge tables for trim, list and also for
temperature. Most LNG carriers are equipped
with process control systems or stand-alone

measurement
systems able to perform these corrections
automatically. It is recommended to use the
millimeter as the smallest unit of dimension,
when applying a tank gauging table.
In some cases the LNG carrier must be
completely emptied after the unloading
operation, e.g. before a long period of inactivity.
In this case a special procedure explained in
Section 3.2.6 is followed for determination of
the volume transferred.
Before loading operations, the LNG carrier may
be in “ready-to-load” condition or otherwise,
may require gassing-up and/or cooling down
operations. In this case a special procedure
explained in Section 2.6 is followed for
determination of the energy and volume
transferred.
Unless parties explicitly agree otherwise (see
below), gas flow is stopped and appropriate gas
valve(s) to engine room shut and sealed during
and between the opening and closing custody
transfer surveys.
The possibility of using LNG and/or boil-off gas
as fuel for the ship during transfer is considered
in Sections 2.1, 2.3.4 and 12.2.
Since the calculation methods described in this
handbook are based on volumes of LNG and LNG
vapor before and after transfer, any use of LNG,
regasified LNG and/or LNG vapor during the
transfer should be fully accounted for by
correction of VLNG, according to the terms of the
LNG purchase and sales agreement.
Note: In-line measurement of LNG quantity
Coriolis mass flow meters and ultrasonic flow
meters are in use at some (un)loading terminals.
However, at the time of writing, their use as part
of a ship-shore custody transfer measurement
system is not yet conventional. This is mainly due
to their high cost, the inability of these flow
meters to handle high flow rates and “proving”
issues. For small scale LNG transfer operations
these meters can be used as secondary (or even
as primary system), if agreed upon by the parties
in their commercial sales conditions, or just as
(operational) verification for the parties involved.
A further informative discussion can be found in
Appendix 1.
2.2.2. LNG Density
The density of LNG is determined by calculation
from the measured composition of the LNG
transferred and the temperature of the LNG
measured in the LNG carrier's tanks.
2.2.3. LNG Gross calorific value
The composition of the LNG is used to calculate
the gross calorific value.
2.2.4. Energy of the gas displaced
by the transfer of LNG
This energy is calculated according to the
composition and volume of the gas displaced,
and the pressure and temperature of the gas
inside the tanks of the LNG carrier before
loading or after unloading. The calculation
procedure is explained in Section 12.1.

measurement

measurement
2.3. INSTRUMENTS USED
2.3.1. For the determination of
the LNG volume
For the determination of the LNG volume the
following are required:
- the LNG carrier’s calibration tables, including
the main gauge tables for each tank and
different correction tables accounting for list
and trim variances (if any) for the main and
secondary gauging systems, tank contraction
tables for Moss-type and SPB-type cargo
containment systems, and possibly, other
correction tables according to the type of
level measuring devices,
- the equipment for measuring the level of
LNG in the LNG carrier's tanks. Each cargo
tank usually has two level gauge systems
installed, one designated as 'main' or
'primary' and the other as 'secondary'.
Capacitance and microwave (radar) level
gauging systems are widely used as primary
CTS systems onboard LNG tankers, backed
up by a secondary CTS system generally
consisting of a float gauging system,
- recently a laser system (called LIDAR) was
introduced in the industry but at the time of
writing its use is restricted to very few LNG
carriers. A further informative discussion can
be found in Appendix 2. Some very old LNG
carriers still in operation are fitted with
nitrogen bubbler systems, which rely on
good knowledge of the LNG density to give
accurate readings,
- temperature probes distributed over the
height of the LNG carrier's tanks,
- other measuring devices required for
applying the correction factors.
Note: Automated systems
The calculation to determine LNG volume may be
automated by processing the level, temperature
and pressure measurements, taking into account
the above mentioned calibration and correction
tables to produce a report meeting CTS
requirements. LNG carriers may be fitted with
certified custody transfer measurement systems
for this purpose. See Section 3.2.7.
LNG density and gross calorific
value
The determination of the density and the gross
calorific value of the LNG transferred is made on
the basis of the average composition of the LNG
obtained by:
- continuous or discontinuous sampling of
LNG in the LNG transfer line(s) between the
ship and the terminal,
- gas chromatographic analysis,
followed by:
- a calculation based on the average
composition of the LNG, its average
temperature and the coefficients given by
the National Bureau of Standards for the
density [9],
- a calculation based on the average
composition of LNG and characteristics of
elementary components (GCV, molar
volume, molar weight) given by reference
tables or standards for the gross calorific
value.

measurement
At a loading terminal LNG sampling and analysis
are made in the LNG transfer line(s) prior to
possible flashing (vaporization) in the ship’s
cargo tanks. If flashing occurs in the ship’s cargo
tanks, then this causes a minor change in LNG
composition since the most volatile components
(typically nitrogen and methane) are
preferentially vaporized and returned to shore
via the vapor return line. Therefore, this effect
should be avoided if possible, or otherwise
minimized, e.g. by ensuring that the tank
pressure in the ship’s cargo tanks is sufficiently
higher than the saturated vapor pressure of the
LNG being loaded.
Notes for information only:
A novel LNG analysis method
At the time of publication of this fifth edition, a different
measurement device for analysing LNG composition based on
direct analysis in the LNG transfer line(s) hence eliminating the
need of an LNG sampling device, vaporizer and gas analyzer, is
being tested in a few pilot applications (see Section 7.6 Raman
spectroscopy).
the energy of displaced gas
The energy of the displaced gas can be
determined from:
- sampling of the gas displaced,
- a gas chromatographic analysis of this
sample gas, enabling the GCV to be
calculated,
- pressure and temperature measurements
within the LNG carrier's tanks.
However, for the determination of the energy
displaced, some parameters such as pressure,
gas composition and temperature can be
estimated from experience and taken as
constant for both custody transfer surveys
before and after (un)loading.
For instance, the displaced gas may be assumed
to be a fixed mixture of nitrogen and methane,
or pure methane. This assumption will hardly
increase the overall uncertainty.
the energy of “Gas to engine
room”
Parties may explicitly agree to allow gas
consumption in the LNG carrier’s engine room
(also including the gas burnt by the ship’s GCU)
during the time between the opening and
closing custody transfer surveys (CTS’s). This
could be to ensure low air-emission operation in
the engine room whilst at berth and so may
favor the use of boil-off gas perhaps
complemented by regasified LNG rather than
fuel oil in the engine room. This practice may
enable the LNG carrier operator to comply with
MARPOL Annex VI (revision October 2008).
For (re)loading operations or ship-to-ship LNG
transfer operations, this could also be done on
the LNG carrier(s) in order to handle the boil-off
gas (flash gas) produced during such operations,
hence reducing the vapor returned to shore or
to the vessel being unloaded.
It is recommended in this case that the LNG
carrier has proper measuring equipment on
board and procedures accepted by both parties
to accurately measure the gas energy
consumption in the engine room between the
opening and closing custody transfer surveys
(CTS’s), and that this on-board gas energy
consumption is taken into account as “Gas to
Engine Room” as per the general formula in
Section 2.1. However, for simplicity the parties
may make a commercial decision to mutually
agree to a fixed gas quantity/volume.

measurement
2.3.5. Periodic instruments
recalibration
It is recommended that, unless it is specified by
the fiscal authorities or by the Classification
Society, Buyer and Seller agree on the
periodicity of recalibration intervals, e.g. at each
dry-docking.
2.4. STANDARDIZATION
International standards exist for the classical
methods and techniques used for LNG Custody
Transfer such as ISO 6976 for calculation of the
GCV of gas.
On the other hand, a number of existing LNG
supply, shipping and purchase agreements
specify GPA 2261 for gas chromatography and
HM 21 or GPA 2145 and GPA 2172 for the
calculation of the GCV of the gas. Buyer and
Seller may approve one of these editions, usually
the most recent edition.
As far as methods and techniques dealing with
static measurement procedures for LNG are
concerned, it should be noted that ISO has
issued numerous international standards, (see
ENCLOSURE 2). The recommendations included
in these documents and future international
standards are appropriate to be considered for
new agreements.
2.5. PARTIAL LOADING OR
UNLOADING OF LNG
CARRIERS
In recent years there has been a worldwide
increase in short term and spot cargo LNG
trading, involving two new operating trends in
LNG shipping:
- more and more LNG shippers are using LNG
carriers as floating LNG storage,
- several LNG shippers have considered and
some have carried out partial unloading
and/or partial loading of one or several
cargo tanks of LNG carriers.
When performing such operations, due
attention should be given to:
- safe ship/shore operating practices and
procedures,
- proper LNG ship/shore custody transfer
procedures.
Please refer to Appendix 3 for recommended
safe practices for partial (un)loading.
2.6. Gassing-up and cooling
down operations
2.6.1. Gassing-up operations
When an LNG vessel is delivered or after dry
dock, the cargo tanks are often filled with inert
(exhaust) gas. As inert exhaust gas contains
carbon dioxide which will freeze during loading,
it must be replaced with warm LNG vapor prior
to cooling down the tanks in preparation of
loading. This process is called gassing-up and is
normally preceded by a process of drying and
inerting.
Drying can be done with hot air (or nitrogen) and
inerting is performed to remove the oxygen out
of the cargo tanks and replace the air/oxygen by
inert gases (exhaust gases of the ship or
nitrogen). In case drying and inerting are
performed with nitrogen, both steps can be
combined in one operation. The reason behind
the preliminary operations of drying and inerting
is not to directly replace air by natural gas due to
safety reasons (hence avoiding an explosive
atmosphere during the operation).

measurement
Once inerted, the shore will supply LNG which is
sent to the main vaporizer of the ship to
produce vapor warmer than the dew point
temperature in the cargo tank. This vapor is then
injected at the top of the cargo tank to displace
the inert gas. The increase of pressure and the
difference in density forces the inert gases out of
the cargo tanks. The exhaust gas is generally
directed to the ship’s (forward) vent mast, to a
vent on shore or burnt in the terminal’s flare (at
the beginning it is fully inert and gradually
becomes a mixture of nitrogen and inert gases
(which percentage is decreasing) and natural gas
(which percentage is increasing). This process
continues until the exhaust gas is measured to
have approx. 98-99 % of methane and the CO2-
content is less than a certain low threshold (e.g.
0.1 %). Once this is accomplished, the vessel is
ready to receive cold vapor and start the cooling
down process.
The quantity of LNG required for this operation
depends on the size and construction of the
vessel’s cargo tanks. The gas quality considered
for gassing up is usually the same as the quality
for cooling and for loading, but some
loading/reloading facilities can use different gas
qualities for each operation. The energy
required for gassing-up the cargo tanks is stated
in the terminal rules or in the contract between
the parties, but the most commonly used
technique is to use the certified gassing-up
tables of the LNG vessel.
The certified gassing-up tables apply a volume of
natural gas which is between 1.7 and 2 times
(each LNG vessel has a coefficient) the volume of
each cargo tank, which is the theoretical volume
of gas to be supplied to each cargo tank for the
purpose of gassing-up this cargo tank.
Each vessel will have a gassing-up table for each
tank on board. These are provided by the tank
manufacturer and confirmed by an independent
surveyor during construction at the shipyard.
Each table is designed specifically for that
particular type of containment system. For
example, a 145 000 m³ size vessel will require
approx. 420 m3
of LNG and 2 850 MWh for a
complete gassing-up operation. This gassing-up
operation will take approx. 20 hours to complete
depending upon the LNG supply and the
vaporization rate on the vessel. The gassing-up
tables should be reviewed and agreed upon by
all parties at the preliminary meeting prior to
commencing any operations.
There are some terminals which correct the
theoretical volume taking into account the
measurement conditions when these are
different than the ones in the certified gassing-
up tables.
Other alternatives to measure the energy for
gassing-up (especially for old LNG vessels which
do not have certified gassing-up tables) are the
following ones:
a) the procedure stated in the LNG vessel
Operations Manual,
b) to apply twice the theoretical volume of
each cargo tank to obtain the theoretical
energy for gassing up,
c) to measure the delivered LNG (in cubic
meters) during the gassing-up operation
by means of an LNG mass or flow meter,
d) to measure the difference in level in the
shore LNG tanks. In this case, it is
necessary to take into account any other
filling (e.g. cold circulation) or emptying
(e.g. send out for regasification
purposes) operations of the tanks.
2.6.2. Cooling down operations
Cooling down operations are performed to
slowly reduce the temperature of the cargo
tanks close to that of the LNG to be loaded in
order to avoid any structural damages by

measurement
thermal shock or stress to the tank construction.
The target is to reach a certain reference
temperature according to the performance
criteria stated in the operations manual or cool
down tables of the LNG vessel.
Cooling down operations are generally
performed on an LNG ship of which the cargo
tanks are under (warm) natural gas (e.g. after a
long ballast voyage during which the cargo tanks
have been warmed up or (immediately) after
gassing-up operations). However it is also
possible to cool down an LNG ship of which the
cargo tanks have been fully dried and inerted
with nitrogen, hence avoiding gassing-up
operations.
LNG is supplied by the LNG terminal and it is
vaporized and injected at a controlled rate into
the ship’s cargo tanks through spray nozzles at
the top of the cargo tanks to avoid thermal
shock in the tanks and its devices (such as
pumps, pump columns, probes…). Once the LNG
is vaporized, there is an increase of pressure in
the cargo tanks and the gas return is sent to the
terminal’s vapor return system or to the
terminal’s flare to keep the pressure in the cargo
tanks under control. The LNG vessel can also
help reducing its cargo tank pressure by burning
gas as fuel (in the engines or in the gas
combustion unit).
The required temperature needed before
loading can be started is identified in the
Operations Manual or the cool down tables
provided by the tank manufacturer. Each cool
down table is specific to that tank type, is issued
by the manufacturer during construction and is
verified by an independent surveyor prior to the
vessel being delivered. The cool down tables
identify the quantity and length of time required
to complete the operation prior to loading. The
gas quality considered for cooling down is
usually the same than the quality for loading
(and gassing-up if there is any), but some
loading/reloading facilities can consider different
gas qualities for each operation. The energy
required for the cooling down operations of the
cargo tanks is stated in the terminal rules or in
the contract between the parties, but it is usual
to apply the use of the cool down tables or one
of the following alternatives.
Cooling down tables
The most common method is the use of the cool
down tables. Based on the size of the cargo tank,
the manufacturer calculates how many cubic
meters of LNG or what energy content are
required to lower the temperature inside the
tank one degree Celsius. Then based on the
vapor temperature inside the tank at the
beginning of the operation, the quantity is
calculated accordingly. The time it takes to
complete a cooling down operation depends on
the temperature prior to starting the operation.
The cooling down tables should be reviewed and
agreed upon by all parties at the preliminary
meeting prior to commencing any operation.
Cooling down operations are generally faster
than gassing-up operations. It takes approx. 10
to 12 hours for a membrane type LNG vessel and
approx. 20 hours for a Moss type vessel.
a) Membrane type LNG vessels
In this type of vessel the reference temperature
is the average temperature (vapor phase) of the
pump tower in each tank excluding the first top
or two top sensors (depending on terminal
rules/contract between the parties).
The reference temperature for loading a
membrane type LNG vessel shall be
approximately -130 °C.

measurement
It is usual to use the certified cooling down
tables. These tables give the energy required to
cool down each tank from its arrival
temperature to -130 °C.
The certified cooling down tables have been
made up supposing a certain gas type in order to
obtain the energy. Once cooling down energy
has been obtained, it is possible to obtain an
LNG cubic meter equivalent for this cooling
down operation.
The tables are divided into two sections:
- Warm conditions. These are used in case the
average temperature in the cargo tanks is
higher than -40 °C. The table normally spans
the range between +40 °C and -130 °C. In
this case the operation can take about 10 to
12 hours.
- Cold conditions. These are used in case the
average temperature in the cargo tanks is
equal to or lower than -40 °C. The table
normally spans the range between -40 °C
and -130 °C. In this case the operation can
take about 6 to 8 hours.
b) Moss type LNG vessels
In this type of vessel the temperature reference
is the equatorial temperature of the cargo tanks.
The reference temperature for loading a Moss
type LNG vessel normally spans the range
between -110 °C and -130 °C depending on the
operations manual of the LNG vessel.
In this case, the certified cooling down tables
could give:
- the LNG volume needed to reduce the
equator temperature of the tank by one
degree Celsius. The total LNG quantity
required for cooling down the cargo tanks
will be calculated by multiplying the
difference between the initial equator
temperature and temperature reference by
the value of cubic meters that are needed to
lower the equator temperature by one
degree Celsius.
- the LNG volume and energy needed to reach
the reference temperature (as the
equatorial temperature).
Nozzle pressure
The cooling down operations are performed by
injecting LNG through special spray nozzles into
the cargo tanks. The flow is fully dependent on
the applied pressure. The number of nozzles, the
average pressure and the duration of the
spraying for each tank can be used to determine
the volume of LNG used for the cooling down
operations.
In case the cooling down operations are
performed on an LNG ship of which the cargo
tanks are under nitrogen, the cool down tables
may be used as well, however this should be
agreed upon by all parties.

3.Volume measurement
3. Volume measurement
3.1. GAUGE TABLES
3.1.1. Use of gauge tables
The gauge tables are numerical tables which
relate the height of the liquid in an LNG carrier's
tank to the volume contained in that tank. The
volume may need to be corrected taking into
consideration various factors.
An independent surveyor usually produces the
gauge tables during the building of the LNG
carrier. They take into account the configuration
of the tank, its contraction according to the
temperature of the liquid and the volumes
occupied by various devices, e.g. cargo pumps.
The calibration tables are usually divided into:
- main gauge tables: height/volume
correlation in ideal conditions,
- correction tables taking into account actual
conditions of the LNG carrier and its
measuring instruments.
For each LNG carrier there is one main gauge
table per tank. Generally the volumes are given
for heights varying cm by cm, the volume for
intermediate heights in mm being calculated by
interpolation. An example of a gauge table is
given in Appendix 4 (Table A4-1).
To avoid these interpolations, which can be a
source of inaccuracy, the most-used parts of the
gauge tables – i.e. heights between 10 and 60
cm and heights corresponding to a volume
between 95 % and 98 % of the total volume of
the tank – are sometimes developed in a fine
gauge table and the volumes can be calculated
mm by mm. This then reduces the
determination of the volume to a mere reading
in a table (see Appendix 4, Table A4-2 and Table
A4-3).
The examples used in this section are taken from
a vessel with prismatic cargo tanks. The same
principles generally apply to those vessels with
spherical or other shapes of cargo tanks.
Various methods exist for establishing the gauge
tables. The main methods are:
- macro metrology with tapes,
- a laser measuring system,
- a photogrammetric measuring system.
For details of calibration procedures for tanks,
reference can be made to existing ISO standards
(see ENCLOSURE 2).
3.1.2. Correction tables
The gauge tables are completed with correction
tables established according to:
- the condition of the LNG carrier (trim/list),
- the average LNG temperature in the tank
that influences contraction or expansion of
the tank,
- the temperature in the gaseous phase,
and/or the density of the LNG influencing
the level measuring devices.
Tank gauge tables may also provide an example
of how to conduct the volume calculation using
the measurements provided.
It should be noted that LNG carriers normally
have two level measurement devices in each
cargo tank (and often of two different types).
Correction tables are specific to a level gauge.
Using the correction tables for the wrong gauge
can result in significant inaccuracies.

3.1.2.1.Correction according to the
condition of the LNG carrier
The gauge tables are established for an LNG
carrier with zero list and trim. Therefore, it will
be necessary to correct the height reading to
take into account a list or a trim which is not
zero.
Correction tables are made up according to:
- the position of the gauge in the tanks,
- the list of the LNG carrier (see Figure 1),
- the trim of the LNG carrier (see Figure 2),
These corrections can be positive or negative. So
the real height will be equal to the algebraic sum
of the height reading, the correction for list and
the correction for trim. These tables are made
up in degrees for the list and in meters for the
trim, with fixed steps of variation. For
intermediate values, the correction will be
calculated by interpolation.
In practice, the LNG carrier's cargo officer will
usually manage the vessel's ballast to obtain
zero or a limited list and trim.
3.1.2.2.Corrections according to the
temperatures in the liquid and gaseous
phases
The corrections are related to the volume
variations resulting from the contraction of the
tanks and their insulation according to the
temperature of the liquid and gaseous phases.
This phenomenon is significant for LNG carriers
with self-supporting tanks. Appendix 4 gives an
example of these tables (see Table A4-4).
3.1.3. Approval by authorities
The gauge tables may be approved by either the
authorities of the countries concerned with the
LNG sale and purchase or by independent sworn
measurers. In practice, largely due to the
importance for LNG shipping of the Japanese
market and its requirement for NKKK
certification, the great majority of LNG carriers
have such certification.
This approval may be valid for a limited duration,
generally 10 to 12 years, or less depending on
LNG terminal requirements, provided there are
no modifications to the tanks. For the European
Union, this approval corresponds to a
community directive.
When an LNG carrier is put into operation, a list
of all works on the tanks must be supplied, and
the tanks must be inspected for any
modifications which might affect the volume.
In the case of any distortion or modification to a
tank, the gauge table must be adjusted
accordingly.
The applicable and confirmed standards
correspond to:
- ISO 8311:2013 Refrigerated light
hydrocarbon fluids – Calibration of
membrane tanks and independent prismatic
tanks in ships – Physical measurement,

FIGURE 1
List represented by the angle α in degrees to port. In this illustrative case,
the correction will be negative.
FIGURE 2
Trim expressed in meters or fractions of a meter, according to the difference
in bow and stern drafts.
The correction is negative or positive depending on whether the bow of the
LNG carrier is deeper in the water or otherwise. In this illustrative case the
correction will be positive.

3.2. INSTRUMENTS AND
METHODS FOR
MEASURING THE LEVEL OF
LIQUID IN THE LNG
CARRIER'S TANKS
3.2.1. Main liquid level gauging
devices
The main types of gauges are:
- electrical capacitance type gauge,
- float type gauge,
- radar type gauge,
- laser type gauge: a recently developed
system.
Currently ISO 18132-1:2011 and ISO 18132-
2:2008 are valid for the above-mentioned level
gauging devices.
Any of these gauging systems can serve as main
instrument for liquid level measurement inside
an LNG cargo tank. Usually (but not always) two
or sometimes even three types of the above-
mentioned systems are installed in each cargo
tank of the LNG carrier. One of these should be
agreed as the main (primary) level gauging
device by buyer and seller. The standby or back-
up gauging system(s) is (are) to be considered as
the auxiliary (secondary) gauge systems.
A few LNG shipping agreements do not specify
the type and only specify the required accuracy
(e.g.  5.0 mm or better). On old ships other
systems may be found, such as nitrogen
bubbling devices, but the accuracy of these is
generally lower and requires a good knowledge
of the actual LNG density in the cargo tank.
Accuracy may be verified at the time of gauging,
as follows:
- for electrical capacitance type gauges,
by an on-line validation system,
- for radar type gauges, depending on the
design, either by a verification pin or by
comparison with the determined length
of each still pipe segment,
- for float type gauges, either by
comparison with the other gauging
system or the stowed/grounded
instrument’s level readings.
For the measurement of liquid levels in terminal
berths exposed to the open sea, filtering
systems approved and accepted by sworn
surveyors as suitable to be integrated in CTS
operations may be considered. These filtering
systems were specifically developed for offshore
use where large level fluctuations may be
experienced. ISO 10976:2012 includes
information on these systems.
3.2.1.1.Electrical capacitance type level
gauge
The electrical capacitance gauge (see Figure 3)
consists of two concentric tubes made of
aluminum or stainless steel, depending on the
construction of the LNG cargo tank. The inner
tube is supported by the outer tube by means of
concentric insulators placed at regularly spaced
intervals along the whole length of the tubes.
The resulting assembly forms a series of
cylindrical capacitors, having the same total
height as the cargo tank of the LNG carrier.
The LNG, according to its level, will fill the space
between the concentric tubes. The liquid affects
the dielectric characteristics of the capacitors
formed by the tubes such that, by measuring the
change in capacitance, the height of the LNG in
the annular space, and hence the level in the
tank, can be determined. The contraction of the

tube assembly at low temperature may be taken
into account to correct the level measurement.
The accuracy of the measurement resulting from
the calibration of the dimensions and the
linearity of the capacitor and of the electronics
should be, for the gauge as a whole, ±5.0 mm
[1].
3.2.1.2.Float type level gauge
Measurements are made with a float hanging on
a tape or a ribbon (see Figure 4). According to
the level of the liquid, the float is displaced, and
the tape or the ribbon on which it hangs is
unrolled or rolled up on a drum whose rotation
is recorded. This enables the position of the
probe, and thus the level of liquid in the tank, to
be known.
With float gauges, it is necessary to take into
account the shrinkage of the ribbon, according
to the temperature of the gaseous phase and
the height of the liquid, and the density of the
LNG, which will influence the float buoyancy.
The correction tables will tabulate the
corrections for these effects.
The corrections for temperature are required
only in the case of a stainless steel ribbon. In the
case of an invar ribbon, the shrinkage is much
less and is generally considered as negligible.
The accuracy of this type of gauge, designed for
marine application, is in the range of ±4 mm to
±8 mm.
3.2.1.3.Radar (microwave) type level gauge
The radar or microwave type gauge works on
the same principle as a ship's radar (see Figure
5). A transmitter/receiver is mounted on the top
of the cargo tank and emits microwaves
vertically down towards the surface of the liquid.
The signal is reflected from the surface, received
by the transmitter's antenna and sent back to
the control panel. The signal is then processed to
determine the distance of the liquid surface
from the transmitter and hence ullage. One or
more certified transmitters/ receivers are
usually carried as spares in the event of a failure
of this equipment.
Since all the level detection components are
mounted external to the cargo tank, the
microwave system allows for the possibility of
changing the gauge in service.
The radar gauge requires a wave guide which is
like a stilling well. However, this well is a critical
and complex component resembling a gun
barrel.
There are several types of gauging pipes used for
microwave type CTS level gauges. Particular
attention should be paid to bottom attenuators,
which can limit minimum gauging height and
therefore accuracy due to their mountings.
The accuracy of this type of gauge can be ±5.0
mm or better.
3.2.1.4.Laser type level gauge
The laser based sensor technology (LIDAR) is a
recently introduced gauge type (see
20.Appendix 2:).
The laser unit is installed above the deck,
isolated from the tank by a sight glass and
targeting the liquid level surface with a low
power laser beam. This beam is protected from
adverse influences on its measurements arising
from within the tank by an ordinary stilling well.
The stilling well used on a laser based system is
not used as a wave guide, its construction is
simple and it remains relatively inexpensive.
The time interval between the transmitted and
received pulses is accurately measured and
processed to determine the distance of the

liquid surface from the transmitter and hence
the ullage height.
The level measurement remains unaffected by
changes in the LNG composition or by the
changes in physical properties of the still pipe.
When performing level measurements at the
bottom of the tank, echoes or reflections near
the bottom do not affect the accuracy of the
system. The gauge will also register a dynamic
change in the LNG level. The accuracy of this
type of gauge is ± 7.5 mm or better.
3.2.2. Timing of the level
measurement
General guidelines are described in the standard
ISO 6578 – Refrigerated hydrocarbons liquid –
static measurement – calculation procedure.
It is important that the level readings are
recorded in a static and closed condition of the
LNG carrier, with no flow of either gas or liquid.
As far as practicable, the period between the
time of measurement and start of cargo transfer
should be narrowed to the minimum achievable.
The same should apply for the final cargo
custody transfer level measurement, once time
is allowed to correctly drain cargo lines (as
appropriate) and stabilize the liquid levels. The
same temperature and inventory condition of
the ship’s LNG cargo lines should be maintained
during both opening and closing custody
transfer surveys. Wherever it is intended that
the vessel heels out (is completely emptied) and
cargo lines are empty upon completion of
discharge, it is necessary to ensure that the
cargo lines are drained for sufficient time prior
to the closing CTS, to ensure stabilization of the
temperature of the lowest temperature sensor.
3.2.3. Readings
It is good practice that all readings are witnessed
by both parties, buyer and seller. Each party
should appoint a representative. A sworn
surveyor may also be jointly nominated by both
parties to stand as third and neutral party, and
witness and record the CTS readings.
The readings of the levels of liquid in the tanks
are taken after the readings of the list (port or
starboard), and the trim (bow or stern) of the
LNG carrier. All these readings will be taken into
account in order to determine the corrected
liquid level and hence the LNG volume in each of
the cargo tanks. The temperatures of the liquid
and the gaseous phase are also measured (see
Section 4 and Section 4.2). The absolute
pressure in each LNG cargo tank is measured as
well. If the pressure measurement is gauge
instead of absolute, the atmospheric pressure is
also read.
Parties should also agree in advance what
actions are to be taken in the event of a failure
of any part of the volume measurement system
(e.g. consider secondary level gauging, manually
calculate volumes with tank tables, etc.).
3.2.3.1.Reading of the level with float
gauges
It is good practice that all float gauges are
lowered to the liquid level well before any
reading is taken. This should allow for sufficient
time to cool down either the gauge ribbon or
wire and to ensure shrinkage, if any, is stabilized.
It is also good practice to verify that the
calibration seals on the float gauge unit are
intact.
Float gauges normally have two reference
readings, upper and lower. The upper reading
can always be checked. Depending on the
manufacturer, this corresponds to a locked
position of the float gauge in its stowed
condition or a resting position of the float on the
valve’s isolation gate. These reference readings
are reported on the ship’s CTS calibration

certificate and they are also stamped on a plate
fixed to the gauge itself. It is advisable that the
upper reference is checked before the cargo
tank liquid levels are taken.
The cargo liquid level is never still and a slight
movement of the float is always to be expected.
Even when the ship is not subject to any wave
motion, the readings will always be an average
level between upper and lower peaks of level
readings.
If no fluctuation of the liquid level is detected,
this may indicate a possible “sticking” of the
float inside the gauge well. If this is the case, it is
recommended that the float is moved up and
down a number of times in order to free it and
obtain reliable readings.
On occasions when the LNG carrier’s tanks are
completely empty, the lower reference of the
float can also be checked for correctness. This is
usually called the float’s grounded position.
Whenever remote float readings are available in
the ship’s control room, it should be checked
that these are approved for commercial
transactions before they can be used for CTS
purposes. Float gauges are always stowed in
their fixed upper position when sailing so that
the ribbon or the tape does not break due to
liquid movements.
3.2.3.2.Reading of the level with capacitance
and radar gauges
The methods of reading are normally agreed
between the seller and buyer. Usually the
observed cargo tank level is averaged over 5
readings taken at specified and regular intervals.
The above method may not be suitable for LNG
terminal berths exposed to the open sea
because of the high fluctuations of the liquid
levels. To cope with this, some custody transfer
measurement systems manufacturers have
lately developed filtering systems in order to
average the level fluctuations and provide a
realistic liquid level (see Section 3.2.1).
Level readings are normally available in the
ship’s cargo control room and sometimes in an
instrumentation room located in the
accommodation block.
Some modern on-board CTM systems use
computer to process all the information,
including averaging the (5) level readings over
time, temperature and pressure, and draw on
digital gauging tables to produce a printed
document containing all the ship-generated
information required for the custody transfer.
However, these seldom include “gas displaced”
or “gas to engine room”, if applicable.
3.2.4. Correction of readings
3.2.4.1.Float gauge
The readings made on the measurement
appliances should be corrected according to:
- list,
- trim,
- density of LNG, affecting float buoyancy,
- coefficient of contraction of the material and
the insulation of the tanks; this coefficient is
more relevant in the case of self-supporting
tanks (see Appendix 4, Table A4-4),
- temperature of the gaseous phase if the
ribbon or cable is not made of invar.
- standard and fixed corrections if applicable.
The corrections are made using specific tables
for each of the above corrections.
3.2.4.2.Capacitance, radar and laser gauges
In this case, only the corrections for list, trim and
the contraction of the tanks are normally taken
into consideration. For accurate level
measurement the contraction of the capacitance

gauge at low temperature may also need to be
considered.
Two main types of radar (microwave) gauges are
currently available. One type is based on the
principle of estimated velocity of the
microwaves inside an atmosphere of
predetermined hydrocarbon composition.
Within the worldwide range of commercially-
available LNG compositions this has little impact
on the overall accuracy of the gauge except for
nitrogen content in the vapor which may
significantly alter wave velocity and should be
considered.
One available type of microwave gauge uses
several defined targets made of PTFE (Teflon®)
at known positions along the stilling pipe. This
enables the system to precisely calculate the
microwave propagation speed and adapt it to
locate the liquid level. The PTFE target position is
usually known at +20 °C. In order to determine
the wave propagation speed, the position of the
targets is corrected according to the shrinkage of
the still pipe, which is calculated as a function of
the vapor phase temperature above the liquid
level.
As to correction of readings for laser gauges,
please refer to Appendix 2. Modern computer-
based systems usually can accept trim and list
data either manually or from external sensors
and automatically apply the corrections.
However, it is important to compare the
observed trim/list to the data from the trim/list
independent sensor.
Depending on the sensor’s location, either
sagging or hogging of the LNG carrier may
seriously affect the accuracy of the readings.

FIGURE 3: ELECTRICAL CAPACITANCE TYPE LEVEL GAUGE
1. Outer tube
2. Inner tube
3. Concentric electrical insulator
4. Isolation of inner tube sections by a gap or dielectric plug
5. Isolation from the tank bottom
6. Connections between the sections of the outer tube to make a single
electrical conductor.
7. Data signal connections from the gauge to the junction box outside the
cargo tank
8. LNG cargo tank

FIGURE 4: FLOAT TYPE LEVEL GAUGE FIGURE 5: RADAR (MICROWAVE) TYPE LEVEL GAUGE

3.2.5. Use of spare level gauge
When the main or primary level gauge cannot be
used, the spare level gauge, also called the
secondary level gauge, is used to measure the
level of LNG. Therefore it should always be in
operation. Usually, the secondary level gauge
will also be calibrated and certified. However, if
the calibration tables of the LNG carrier are
available only for the main level gauge, a
conversion table is required in order to take into
account the respective locations of main and
secondary gauges, or the statistical differences
between the two level gauge measurements,
and to evaluate the corresponding corrections
which must be applied to level measurement
before using the calibration tables.
Since it is possible that the primary level gauging
system could fail during the cargo transfer
operation, and since it is strongly recommended
to use the same reference system at both
opening and closing CTS measurements to
determine the cargo volume transferred, it is
therefore advisable that both primary and
auxiliary level gauging systems readings are
recorded at the opening CTS measurement,
before the start of cargo transfer operations. It
should also be ensured that the spare level
gauge is maintained within stated accuracy at all
times.
3.2.6. Complete unloading (tank
stripping)
Cargo tank stripping with stripping pumps:
- the LNG carrier needs to be able to continue
discharging once the cargo pumps cannot
longer be kept running because the level in
the cargo tank is too low,
- these stripping operations require several
additional hours at the port. Stripping to
lowest levels is generally conducted prior to
scheduled dry-docking or for spot cargo
operations,
- the remaining quantity after stripping can be
vaporized by warming up to a temperature
at which the LNG carrier is considered to be
empty of LNG. e.g. -80 °C (no more liquid
ethane) or -40 °C (no more liquid propane).
The vaporization of residual LNG is typically
conducted not at berth but at sea, after the
unloading is finished and the vessel has left
the berth.
For the complete or nearly complete unloading
of an LNG carrier with prismatic tanks,
depending on the membrane type, the ship
should be trimmed to the best condition to
achieve effective stripping and a reliable level
reading at the end of operations. In general, LNG
carriers with prismatic tanks require the ship to
be trimmed by stern in order to achieve
complete or nearly complete unloading of cargo.
Depending on the membrane type, higher trim
levels will also result in more effective cargo
tank stripping provided that care is taken not to
exceed neither ship nor Terminal limitations, if
any in place. Whatever the primary membrane
configuration, some LNG liquid will, however,
always remain in the cargo tank and that needs
to be quantified. It is essential that, as much as
possible during the stripping operation, the
liquid level is always within the certified
minimum gauging height. e.g. using onboard
CTMS or cargo tank tables using appropriate
correction tables. In the event the liquid level
falls outside the certified minimum gauging
height, buyer and seller should agree on the
immeasurable cargo quantity.
The energy of this remaining LNG transferred
either in the liquid or in gaseous form can also
be determined by mutual agreement by the
parties taking into account any technical
limitations they may have.

Note
Certain parties have raised concerns over the
possibility of having a quantity of LNG
unaccounted for during closing of CTS under
a positive trim as a result of LNG being
trapped in the bottom part of uncovered
corrugations of MARKI/MARKIII membrane
systems, where small liquid wedges may form
between the tank’s bottom plate and the
transverse corrugation in each of the exposed
tank’s bottom cells. The validity and extent of
such liquid entrapment is currently unknown.
This will require further and full investigation
by independent and expert parties, before
any conclusion can be made on the subject.
The conclusion of such investigation should
then be fully endorsed and certified by
independent and recognized sworn
surveyors.
3.2.7. Automated systems
The LNG carrier may be fitted with an automatic
system for the calculation of LNG and gas
volumes in each tank. The use of such a system,
commonly referred to as the ship’s custody
transfer measurement system (CTMS) will
facilitate the process of determining quantities
transferred during loading and unloading.
The CTMS processes data from tank level,
temperature, pressure sensors, etc. in real-time,
taking into account the required corrections and
certified gauge table, to produce a calculation of
volumes before, during and after transfer. By
taking measurements frequently, data can be
averaged to improve repeatability of the
calculation.
Modern custody transfer measurement systems
typically comprise two discrete parts: (a) the
tank gauging system providing corrected tank
levels, temperatures and pressures and (b)
workstation(s) and peripherals, usually located
in the ship’s cargo control room, to perform the
volume calculation and generate reports.
Prior to such systems being entered into service,
the calculation, including corrections and gauge
tables programmed into the system, should be
certified as accurate by an independent and
competent third party, see Section 3.1.3. Once
the software has been verified, the calculation
method may be regarded as reliable so long as
the software is unchanged. Software
modification may necessitate re-certification. It
should also be recognized that the sensors have
to be recalibrated at agreed intervals to ensure
that input data is accurate, see Section 2.3.5.
Custody transfer measurement systems may be
capable of correcting for trim and list both
manually (by operator input of draught readings)
and automatically (by sensor data). The method
should be agreed between buyer, seller and
terminal operator. Trim and list sensors cannot
be readily recalibrated once in situ so manual
methods are often preferred. Due attention
should be given to the proper location of the
trim/list gauges on board the vessel. If due to an
inappropriate location these gauges are
influenced by the position of the vessel, this may
affect the accuracy of the instrument.
It is appreciated that a software-based
calculation may be less transparent to the buyer,
seller and terminal operator than more
traditional methods. The CTMS as a whole
should be dependable in use. In addition to
software verification, due regard should be given
to hardware availability, reliability and
maintainability to minimize the likelihood and
consequence of failures in use.
It is recommended that instruments are
connected directly to the system, i.e. sensor
data cannot be manipulated by other systems
unless part of the certified arrangement.
Furthermore, computers (PC, process
controllers), data communication links (serial,

network) and peripherals (screens, keyboards,
printers) should not, in general, be shared with
other applications in order to maintain data
integrity. However, a copy of the calculation
software may be hosted on a shared workstation
as a back-up to the primary system.
Data used in the transfer process, such as tank
levels, may be transmitted from the CTMS to
other ship’s control and monitoring systems
providing this does not influence the custody
transfer calculation. In general, data should not
be transmitted to the CTMS from other systems
unless it is part of the certified arrangement.
Custody transfer measurement systems are
typically able to produce printed reports
indicating volumes before and after transfer.
However, the ability to automate this part of the
custody transfer survey does not obviate the
need to provide, if required, the information
required to carry out the calculation manually.
3.3. CALCULATION OF THE
VOLUME OF LNG
TRANSFERRED
This calculation is illustrated by an example
given in Appendix 5 showing the results of the
volume determination before and after loading
the LNG cargo, with the following assumptions:
- a "Gaz Transport"-type LNG carrier with 5
invar membrane prismatic tanks,
- in each tank, one float gauge with a stainless
steel ribbon.
The example as illustrated is representative of
an LNG carrier with an older level gauging
system. In order to calculate the volume
transferred, two conditions are required,
knowledge of the total cargo volume on board
(a) before the start of cargo transfer operations
and (b) after these cargo transfer operations are
completed. These two conditions are either
manually recorded or directly produced and
printed out by a computer if available. Details of
each cargo tank parameters must be logged. In
order to document a cargo transfer operation,
modern primary custody transfer measurement
systems produce three printouts, a so-called
“before loading/discharge” cargo tanks status,
an “after loading/discharge” cargo tanks status
and a third printout just after the “after
loading/discharge” condition; which is usually
referred to as the “Certificate of
Loading/Discharge”. This third printout
summarizes the general parameters of the two
conditions and shows the volume transferred by
comparing the initial and final cargo volumes. An
example of Custody Transfer Survey documents
for an unloading of an LNG vessel is given in
Appendix 15.
It is essential that the LNG carrier’s pipework
and manifolds are kept as far as practical in the
same inventory condition at the opening and
closing Custody Transfer Surveys.
Figure 6 illustrates the calculation procedure to
determine the volume of LNG transferred based
on the ship’s measurements.

FIGURE 6: FLOWCHART FOR DETERMINING THE VOLUME OF LNG TRANSFERRED
Measured
level
Level
corrections
Corrected
level
Volume
correction
Level-to-Volume
conversion
Volume from
tables
Corrected
Volume
Temperature
Trim
List
Density
Correction
tables
Correction
table
Liquid
temperature
Gauge Tables
Level Gauge
VOLUME
BEFORE/AFTER
(UN)LOADED
VOLUME
 tanks
larger volume -
smaller volume
TANK VOLUME
SHIP VOLUME
VOLUME TRANSFERRED

4.Temperature measurement
4. Temperature measurement
4.1. LIQUID TEMPERATURE
4.1.1. Device
The LNG temperature is measured by probes
placed at different heights in the tanks. These
probes are generally three- or four-wire
platinum resistance temperature sensors
(usually PT100), of which there are typically five
per cargo tank. Recently-built LNG ships often
have also calibrated back-up sensors fitted next
to each main thermal sensor, which can be
selected and used in case of main sensor failure.
One probe should always be in the liquid phase
at the bottom of the tank and one always in the
vapor phase at the top of the tank.
The tank gauge table also provides the vertical
height of the location of the temperature
sensors, so that it may be determined whether
the sensor is in the liquid or in the vapor phase
during custody transfer measurement.
The temperature measurement of these probes
is converted into degrees Celsius with the help
of a data acquisition computer equipped with a
printer. Table 1 shows an example of a printout
of LNG temperatures when the tanks are filled
to 98 % capacity with LNG.
Figure 7 shows a diagram of temperature
measuring devices installed on a LNG carrier. In
this example, five probes are immersed in LNG in
each tank. A sixth probe (not shown) is in the
vapor phase at the top of the tank.
The average liquid temperature is calculated
upon the opening and the closing Custody
Transfer Surveys. The average liquid
temperature is calculated using the temperature
reading at each individual temperature sensor
that is in the liquid, and not the average
temperature of each cargo tank.
Thermocouples are not used for LNG
temperature measurement within custody
transfer because they are less sensitive at very
low temperatures and often give a less accurate
measurement than platinum resistance probes.
In addition their installation is more complex
requiring compensation cables. They may be
installed sometimes for control or simple
indication purposes such as cooling down or
heating of the tank.
4.1.2. Testing and accuracy
The accuracy of the platinum resistance
thermometer varies between ±0.1 and ±0.2 °C
for temperatures ranging between -145 and
-165 °C. The overall uncertainty of the
temperature measuring chain can be estimated
at about ±0.5 °C (including sensor, cable, signal
converter, display).
The sensors are tested for accuracy at the
manufacturer’s facility prior to installation, and
certified/calibrated upon installation in the
cargo tanks. The temperature measurement is
tested and recalibrated at regular intervals to
ensure continued performance.
The influence of temperature measurement
accuracies on the determination of LNG density
(see Section 15) is important. For instance, for
LNG with an average density in the range 440 -
470 kg/m3
, and at a temperature around -162 °C,
the relative accuracy in density calculation, due
to an accuracy of 0.5 °C in temperature
measurement, is about 0.15 %.

4.Temperature measurement
4.2. VAPOR TEMPERATURE
The temperature in the gaseous phase of the
tanks is used to determine the quantity of gas
displaced during the loading and unloading
operations, or the level correction of the float
gauge due to ribbon shrinkage.
As for liquid temperatures, the average vapor
temperature is calculated using the temperature
reading at each individual temperature sensor,
and not the average temperature of each cargo
tank. However, only temperatures indicated by
probes not immersed in the LNG are averaged in
this case.
Typically an accuracy of 1.0 °C (in the range -
145 to +40 °C) is required.
TABLE 1: EXAMPLE OF TANK TEMPERATURE RECORDINGS (98 % FILLED)
TEMPERATURE
PROBE
TANK 1
°C
TANK 2
°C
TANK 3
°C
TANK 4
°C
TANK 5
°C
T 1 -161.83 -161.90 -161.94 -161.89 -161.84
T 2 -161.80 -161.88 -161.91 -161.89 -161.86
T 3 -161.82 -161.87 -161.92 -161.90 -161.90
T 4 -161.79 -161.86 -161.87 -161.88 -161.81
T 5 -161.82 -161.82 -161.88 -161.91 -161.84
TANK AVERAGE -161.81 -161.87 -161.90 -161.89 -161.85
The liquid temperature in each tank is determined by the arithmetic average of temperatures indicated
by the probes that are dipped in the LNG and that are in working order.
FIGURE 7: DIAGRAM OF TEMPERATURE MEASURING DEVICES ON BOARD A LNG CARRIER

5.Vapor pressure measurement
5. Vapor pressure
Vapor pressure measurements can be made
with a pressure gauge, which indicates the
pressure in the gas spaces of the cargo tanks.
This pressure is needed to calculate the energy
of displaced gas, see Section 12.1. For this, it is
necessary for the pressure to be in absolute
terms. If the ship's instrumentation measures
pressure in 'gauge' terms, then the atmospheric
pressure must be recorded and added to the
gauge pressure.
The pressure value is recorded, with the
atmospheric pressure if appropriate, at the time
of taking the other CTS readings.
Typically the required pressure measurement
accuracy is specified as between 0.1 and 1 % FS
(% of full-scale instrument range).
ISO 10976 also addresses vapor pressure
measurement.

6.Sampling of LNG
6. Sampling of LNG
6.1. LNG SAMPLING
PRINCIPLES
In order to determine the quality of the LNG it is
first necessary to undertake particular
operations to condition the fluid sampled from
its initial state, liquid at low temperature, to a
final state, gas at ambient temperature, without
partial vaporization or loss of product.
Sampling of LNG includes three successive
operations:
- taking a representative sample of LNG,
- complete and instant vaporization,
- conditioning the gaseous sample (e.g.
ensuring constant temperature and
pressure) before transporting it to the
analyzer and/or sampler.
Sampling is the most critical point of the LNG
measurements chain. Each step must be taken
without changing the sample composition. This
is by far the most complicated phase of the
measurements and most of the problems
observed in determination of the energy loaded
or unloaded come from the sampling system.
The sampling system is not changeable during
unloading/loading. Some operators have a back-
up sampling system to ensure sample collection
in the event of failure of the main system.
The LNG industry has evolved the sampling
processes. The "spot (discontinuous) sampling
system" described in the first edition of this
handbook has become almost obsolete for CTS
measurements. It is therefore recommended to
use this only as a back-up system in case of
failure of the main device. The spot sampling
system may be used, however, for the (manual)
sampling for impurity analyses. For
completeness, description of this system is given
in Section 6.9.
The sampling processes currently used in the
LNG industry are mainly of two types;
continuous sampling and intermittent sampling,
both sampling processes as defined in ISO 8943.
The intermittent sampling is also referred to as
discontinuous sampling in other publications
such as the EN 12838 standard and the current
GIIGNL handbook. Please note that the
terminology continuous/discontinuous is
different from that used in the first and second
edition of this handbook.
The terms continuous and discontinuous
sampling are related to the analysis of gaseous
LNG, that is, after evaporation of the sampled
liquid stream. LNG sampling systems always
sample LNG on a continuous basis:
- continuous sampling:
This sampling process involves continuous
collection of LNG from the LNG flow line
during the loading/unloading operations.
The regasified LNG from the vaporizer is
thereafter continuously fed into the gas
sample holder. Gas sample containers
(definition according to ISO 8943) are filled
with the mixed gas from this gasholder after
completion of the sampling process for
offline analysis.
- discontinuous sampling (referred to as
intermittent by ISO 8943):
This sampling process also involves
continuous collection of LNG from the LNG
flow line during loading/unloading
operations. However, the regasified LNG
from the vaporizer is in this process partly
directed to an on-line GC and partly into a
constant pressure floating piston (CP/FP)
sample container (definition according to
ISO 8943). A CP/FP sample container is
capable of maintaining constant pressure
during the sampling of gas from the process
line into the gas cylinder. The gas sample

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